I believe you cannot set the source encoding as a project setting. Instead, it tries to determine the source encoding from the file, using a fairly limited algorithm. Part of that algorithm is
- if it has an UTF-8 signature (BOM), it is UTF-8 encoded
- otherwise, it is in the ANSI system code page (CP_ACP)
There might be some additional checks in-between (e.g. checking for UTF-16 with BOM also).
In my experience, the "save as" encoding is not very useful, unless saving as UTF-8-with-BOM. You can save it in a different encoding, but in reopening, it will still assume it's CP_ACP.